Investigations on an H-fractal Wideband Microstrip Filter with Multi-passbands and a Tuned Notch Band

This paper investigates an H-fractal wideband microstrip filter with multi-passbands and a tuned notch band for wireless communication frequencies. The four different filter configurations explored are: symmetric with zero offset, symmetric with nonzero offset, asymmetric with zero offset, and asymmetric with nonzero offset. The effect of H-fractal iterations, fractal scaling parameters, and stub offset on the filter’s multi-passband response is presented. A comparison is made to a non-fractal straight stub filter of equivalent length showing improved passband bandwidth while maintaining the same overall response. Then an asymmetry is introduced into the fractal geometry to produce a tuned notch band in the second passband. Two fractal scaling factors are shown to aid in the tuning of the filter notch band. Finally, an asymmetric filter is fabricated on FR-4 substrate and experimentally verified, illustrating that the filter has multi-passbands and can find applications in WiFi/WiMAX transponders. The fabricated filter’s first two passbands (with respect to S11 = −10 dB) are from 2.09 GHz to 3.18 GHz (fractional bandwidth of 1.09 GHz, 41.36%) and from 4.1 GHz to 5.43 GHz (factional bandwidth of 1.33 GHz, 27.91%), both for WiFi applications along with a notch band (S21 = −3 dB) from 3.3GHz to 3.94GHz (factional bandwidth of 0.64 GHz, 17.67%) to suppress co-site WiMAX transmission. The measured data agrees reasonably well with the simulated filter response.